This invention relates to a hard alloy having excellent hardness, toughness, wear resistance, fracture resistance, plastic deformation resistance and thermal cracking resistance, in which plate-crystalline tungsten carbide (hereinafter abbreviated to "platy WC") is crystallized, specifically to a platy WC-containing hard alloy suitable as cutting tools such as an insert, a drill and an end mill, a base material of a coating super hard tool, plastic working tools such as a drawing mold, a die mold and a forging mold and shearing tools such as a punching mold and a slitter, a composition for forming platy WC and a process for preparing the platy WC-containing hard alloy.
In general, hardness, i.e., wear resistance and strength and toughness, i.e., fracture resistance of a hard alloy can be changed by a particle size of WC, a Co content and an addition amount of other carbide so that the hard alloy can be widely used for various purposes. However, there is a problem of antinomy tendency that if wear resistance is heightened, fracture resistance is lowered, while if fracture resistance is heightened, wear resistance is lowered.
As one course for solving this problem, there may be mentioned a means obtained by paying attention to anisotropy of mechanical characteristics due to crystal faces of WC, specifically, for example, a means relating to a hard alloy in which platy WC exists, which platy WC has a shape represented by a triangle plate or a hexagonal plate and has a (001) face preferentially grown in the direction of the (001) face since the (001) face of WC crystal shows the highest hardness and the direction of a (100) face shows the highest elastic modulus, or a process for preparing the same.
As representative examples of prior art techniques relating to platy WC, there may be mentioned Japanese Patent Publications No. 23049/1972 and No. 23050/1972 and Japanese Provisional Patent Publications No. 34008/1982, No. 47239/1990, No. 51408/1990, No. 138434/1990, No. 274827/1990 and No. 339659/1993.
Among the prior art techniques relating to platy WC, in Japanese Patent Publications No. 23049/1972 and No. 23050/1972, there has been described a process for preparing a platy WC-containing hard alloy by using mixed powder which comprises colloidal tungsten carbide powder containing a porous agglomerate for growing platy WC and powder of Fe, Ni, Co or an alloy thereof.
In Japanese Provisional Patent Publication No. 34008/1982, there has been described a process for preparing twin tungsten carbide in which (001) faces are bonded as a twin face by adding a small amount of an iron group metal salt to mixed powder of strongly pulverized W and C and then carbonizing the mixture under heating.
Further, in Japanese Provisional Patent Publications No. 47239/1990 and No. 138434/1990, there has been described a process for preparing a hard alloy by using, as a starting material, a solid solution of (W,Ti,Ta)C in which tungsten carbide is contained in a super-saturated state and crystallizing platy WC at the time of sintering under heating.
Next, in Japanese Provisional Patent Publication No. 274827/1990, there has been described a process for preparing an anisotropic hard alloy by subjecting a used hard alloy to oxidation, reduction and then carbonization to obtain powder, molding the powder and then subjecting the resulting molded compact to sintering or hot pressing.
In addition, in Japanese Provisional Patent Publication No. 339659/1993, there has been described a process for preparing a hard alloy containing platy WC by subjecting mixed powder comprising WC with a size of 0.5 .mu.m or less, 3 to 40% by weight of a compound with a cubic structure and 1 to 25% by weight of Co and/or Ni to sintering at 1,450.degree. C. or higher.
In the hard alloys or the hard alloys obtained by the preparation processes described in these 8 publications, the growing rate of the (001) crystal face of WC is low, all of the a axis length, c axis length and c/a ratio of the WC crystal are small and the ratio of platy WC contained is low, whereby there is a problem that all of various characteristics of the hard alloy, particularly hardness, wear resistance, strength, toughness and fracture resistance cannot be improved. Also, in the preparation processes, there are problems that it is difficult to control a particle size, it is difficult to heighten the ratio of platy WC contained, said processes can be applied only to a hard alloy in which compositional components are limited, and preparation cost is high.